Reviewed by Danielle Ellis, B.Sc.Sep 30 2024
As they reported in a study in Physical Review Letters, researchers from the University of Manchester searched for hints by comparing the distribution of galaxies to the cosmic microwave background (CMB), the afterglow of the Big Bang. However, they were unable to find any.
Scrutinizing the cosmic microwave background has enabled scientists to search for exotic particles too light to be found any other way. Image Credit: Planck Collaboration/European Space Agency
Scientists have looked for a hypothetical particle known as the dark photon, a possible messenger from a completely new realm of undiscovered particles that might explain the universe’s enigmatic dark matter using existing astronomical data.
Their search limits some dark photon masses, and the technique could be applied to other mass ranges using different cosmological data sets.
This is definitely going to open multiple new avenues for probing [the dark photon].
Jens Chluba, Theoretical Cosmologist, University of Manchester
Dark matter, which accounts for 85% of all matter in the universe and forms vast haloes that seed the formation of galaxies, is one of cosmology’s most perplexing mysteries. Most models assume it consists of only one type of new particle. For example, it could be made up of weakly interacting massive particles (WIMPs) that only interact via gravity and the weak nuclear force. However, so far, neither WIMPs nor another candidate, much lighter particles known as axions, have been discovered.
According to some physicists, dark matter could comprise an entirely new class of particles known as the “dark sector” that interact only weakly with other particles and through their own forces. In the same way that the ordinary, massless photon conveys the conventional electromagnetic force, a massive dark photon could convey a different form of electromagnetism, for example, in the dark sector.
Since one would occasionally be expected to transform into an ordinary photon, the dark photon could function as a messenger from the dark sector even though it would not be the dark matter. Scientists are using particle beams to blast through impenetrable barriers in the lab in an attempt to find dark photons. In a detector located far beyond the barrier, some of the dark photons produced by the collision could transform into ordinary photons.
Observational cosmologist Fiona McCarthy of the University of Cambridge and her associates have now searched for the opposite phenomenon: the transformation of ordinary photons into dark photons.
They have accomplished this, not in a laboratory, but in space, using a catalog of over half a billion galaxies imaged by NASA’s Wide-field Infrared Survey Explorer, which flew from 2010 to earlier this year, and a precise map of the CMB radiation produced by the European Space Agency’s Planck spacecraft, which collected data from 2009 to 2013.
McCarthy noteed that the analysis allowed the researchers to look for dark photons with masses so low that they would be invisible in particle experiments.
As photons from the CMB travel through space, the probability of them becoming dark photons should increase as they pass through the plasma of electrons that surrounds galaxy clusters. McCarthy and her colleagues looked for a loss of CMB photons that corresponded to the directions of galaxy clusters and made the CMB appear splotchy.
Given how famously pockmarked the CMB already is, that posed a challenge. From point to point in the sky, its temperature varies by about a part in 100,000, reflecting minute quantum fluctuations in the ultrahot soup of fundamental particles that populated the early universe. Thus, the investigators needed to search for minuscule amounts of splotchiness layered on top of each other.
They also needed to take into consideration other factors that could skew the CMB. For instance, electrons whirling in far-off galaxies can emit their own microwaves, and CMB photons can change their energy when they bounce off of electrons. Researchers used the fact that each would alter the CMB spectrum differently to filter out such effects.
Ultimately, the team was unable to detect any dark photons. The data could not exclude the particles, but they did place a limit on how strongly dark photons of a specific mass range—roughly 0.2 to 20 quintillionths the mass of an electron—could meld with regular photons, which was nearly ten times more stringent than a previous analysis of the CMB alone.
It would have been a lot more surprising if we had found something.
Fiona McCarthy, Observational Cosmologist, University of Cambridge
Nonetheless, she believes the analysis was worthwhile because it makes use of existing data and techniques.
“We should be looking [for dark matter] everywhere that we can,” McCarthy stated.
According to Chluba, the technique has the potential for growth. The team’s analysis probed a dark photon mass range that is determined by the density of electrons in the cataloged galaxies, which were divided into two cohorts, one averaging 7 billion light-years away and the other averaging 12 billion light-years.
The electron density should be higher in distant, older galaxies, and looking for their isolated effect on the CMB could help scientists search for heavier dark photons, according to Chluba.
According to him, cosmologists might be able to compare the CMB to the distribution of hydrogen gas during the cosmic “dark ages” before galaxies formed.
Chluba concluded, “Looking at archival data and using it in a new, innovative way is always a great step. It is a free lunch.”